Glossary
alternating current
Alternating current (AC) reverses direction at a regular frequency — 60 cycles per second in North America — and is the current a wall outlet supplies.
Alternating current, or AC, reverses direction at a regular frequency: 60 cycles per second in North America and 50 in most of the rest of the world. Its voltage rises and falls with each cycle too, so it is described by its frequency and by a peak or average value rather than by a single steady number.
AC is what a power station's generator produces and what a wall outlet supplies. It won the grid because a transformer can change its voltage cheaply: the voltage is stepped up for transmission so less energy is wasted as heat in the wires, then stepped down again near homes.
The contrasts are where questions land. A transformer works on AC and not on DC, because it needs a changing magnetic field, so an adapter has to convert AC into DC before electronics can use it. Household circuits are AC and are wired so that the branches sit in parallel, each receiving the same voltage. And a generator produces AC, while a battery supplies DC.
Taught in
Related terms
- direct current — Direct current (DC) is current that flows in one direction only, at a steady value. Batteries, solar panels and the electronics inside a device all run on it.
- transformer — A transformer is two coils sharing a core; it changes alternating voltage by the ratio of turns between them — more turns on the secondary steps the voltage up, fewer steps it down.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)